1. Field of the Invention
The present invention relates to a receiving device, a signal processing method, and a program, and particularly to a receiving device, a signal processing method, and a program that can prevent degradation in accuracy of estimating transmission line characteristics as characteristics of a transmission line of an OFDM (Orthogonal Frequency Division Multiplexing) signal.
2. Description of the Related Art
In terrestrial digital broadcasting and the like, OFDM is adopted as a data (signal) modulating system.
In OFDM, a large number of subcarriers orthogonal to each other are provided within a transmission band, and digital modulation such as PSK (Phase Shift Keying) and QAM (Quadrature Amplitude Modulation) that assigns data to the amplitude or phase of each subcarrier is performed.
OFDM divides the transmission band by the large number of subcarriers, and thus has a narrow band per subcarrier (wave) and a slow modulating speed. However, transmission speed of a total (the whole of the subcarriers) of OFDM is not different from that of an existing modulating system.
As described above, in OFDM, data is assigned to a plurality of subcarriers, and therefore modulation can be performed by an IFFT (Inverse Fast Fourier Transform) operation that performs an inverse Fourier transform. The demodulation of an OFDM signal obtained as a result of the modulation can be performed by an FFT (Fast Fourier Transform) operation that performs a Fourier transform.
Thus, a transmitting device for transmitting the OFDM signal can be formed by using a circuit that performs the IFFT operation, and a receiving device for receiving the OFDM signal can be formed by using a circuit that performs the FFT operation.
In addition, OFDM can improve resistance to a multipath by providing a signal interval referred to as a guard interval to be described later. Further, in OFDM, a pilot signal as a known signal (a signal known on the receiving device side) is inserted discretely in a time direction and a frequency direction. The receiving device uses the pilot signal for synchronization and estimation of transmission line characteristics.
OFDM has a strong resistance to multipaths, and is thus adopted in terrestrial digital broadcasting and the like that would be strongly affected by multipath interference. Standards for terrestrial digital broadcasting adopting OFDM include for example DVB-T (Digital Video Broadcasting-Terrestrial), ISDB-T (Integrated Services Digital Broadcasting-Terrestrial) and the like.
In OFDM, data is transmitted in units referred to as OFDM symbols.
FIG. 1 shows an OFDM symbol.
The OFDM symbol is composed of an effective symbol as a signal duration subjected to an IFFT at a time of modulation and a guard interval formed by copying the waveform of a part of the second half of the effective symbol to the head of the effective symbol as it is.
Supposing that the length of the effective symbol of the OFDM symbol, that is, effective symbol length not including the guard interval is Tu [seconds], and that an interval between OFDM subcarriers is Fc [Hz], the relation of an equation Fc=1/Tu holds.
OFDM improves resistance to multipaths by providing the guard interval at the head of the OFDM symbol as shown in FIG. 1.
In terrestrial digital broadcasting, a unit referred to as an OFDM transmission frame is defined, and the OFDM transmission frame is formed by a plurality of OFDM symbols.
For example, in the ISDB-T standard, one OFDM transmission frame is formed by 204 OFDM symbols. A position at which the pilot signal is inserted is determined in advance with the OFDM transmission frame as a unit.
Specifically, in OFDM using the modulation of the QAM system for modulation of each subcarrier, the phase or amplitude of subcarriers of an OFDM signal obtained by performing OFDM of data is affected differently for each subcarrier due to a multipath or the like at a time of transmission.
Thus, a receiving device performs distortion correction that equalizes the OFDM signal received from a transmitting device so that the amplitude and phase of subcarriers of the OFDM signal received from the transmitting device become equal to the amplitude and phase of the subcarriers of the OFDM signal transmitted by the transmitting device.
Specifically, in OFDM, the transmitting device discretely inserts a known pilot signal whose amplitude and phase are determined in advance within transmission symbols (symbols corresponding to subcarriers) forming OFDM symbols. Then, the receiving device estimates transmission line characteristics as characteristics (frequency characteristics) of a transmission line on the basis of the amplitude and phase of the pilot signal, and corrects distortion of the OFDM signal using transmission line characteristic data indicating the transmission line characteristics.
FIG. 2 shows an arrangement pattern of pilot signals within OFDM symbols.
Incidentally, in FIG. 2 (as in FIG. 5 and FIG. 6 to be described later), an axis of abscissas indicates subcarrier numbers identifying subcarriers of the OFDM signal, and an axis of ordinates indicates OFDM symbol numbers identifying OFDM symbols of the OFDM signal.
A subcarrier number corresponds to a frequency, and an OFDM symbol number corresponds to a time.
In FIG. 2, circular marks (white circular marks and black circular marks) represent a subcarrier of the OFDM signal or a transmission symbol (symbol on an IQ constellation as data for modulating a subcarrier on the transmitting device side) forming an OFDM symbol.
The black circular marks in FIG. 2 represent a pilot carrier as a subcarrier of the pilot signal.
The pilot carrier is arranged at a plurality of predetermined positions of the OFDM signal.
Specifically, for example, in the ISDB-T standard, the pilot signal (pilot carrier) is arranged in every four OFDM symbols (OFDM symbol numbers) in a time direction, and is arranged in every 12 subcarriers (subcarrier numbers) in a frequency direction.
FIG. 3 shows the configuration of an example of an existing receiving device for receiving the OFDM signal.
The receiving device in FIG. 3 includes an antenna 101, a tuner 102, a BPF (Band-Pass Filter) 103, an A/D (Analog/Digital) converter section 104, a quadrature demodulating section 105, an offset correcting section 106, a symbol timing regenerating section 107, an FFT section 108, a transmission line characteristic estimating section 109, a transmission line distortion correcting section 110, and an error correcting section 111.
The antenna 101 receives the broadcast wave of an OFDM signal transmitted (broadcast) from a transmitting device of a broadcasting station not shown in the figure, converts the broadcast wave into an RF (Radio Frequency) signal, and supplies the RF signal to the tuner 102.
The tuner 102 includes an arithmetic section 102A and a local oscillator 102B. The RF signal from the antenna 101 is supplied to the arithmetic section 102A.
In the tuner 102, the local oscillator 102B oscillates the signal of a sinusoidal wave of a predetermined frequency, and supplies the signal to the arithmetic section 102A.
Further, in the tuner 102, the arithmetic section 102A multiplies the RF signal from the antenna 101 by the signal from the local oscillator 102B, and thereby frequency-converts the RF signal into an IF (Intermediate Frequency) signal. The arithmetic section 102A supplies the IF signal to the BPF 103.
The BPF 103 filters the IF signal from the tuner 102, and supplies the result to the A/D converter section 104. The A/D converter section 104 subjects the IF signal from the BPF 103 to A/D conversion, and supplies the IF signal as a digital signal obtained as a result of the A/D conversion to the quadrature demodulating section 105.
The quadrature demodulating section 105 subjects the IF signal from the A/D converter section 104 to quadrature demodulation using a carrier of a predetermined frequency (carrier frequency). The quadrature demodulating section 105 outputs a baseband OFDM signal obtained as a result of the quadrature demodulation.
The OFDM signal output by the quadrature demodulating section 105 is a signal in a time domain before an FFT operation (immediately after a transmission symbol on the IQ constellation is subjected to an IFFT operation on the transmitting device side), and will hereinafter be referred to also as an OFDM time domain signal.
The OFDM time domain signal is a complex signal represented by a complex number including a real axis component (I (In Phase) component) and an imaginary axis component (Q (Quadrature Phase) component).
The OFDM time domain signal is supplied from the quadrature demodulating section 105 to the offset correcting section 106.
The offset correcting section 106 corrects the OFDM time domain signal from the quadrature demodulating section 105 for a sampling offset (a shift in sampling timing) in the A/D converter section 104 and for an offset of frequency of the carrier of the quadrature demodulating section 105 (a shift from frequency of the carrier used in the transmitting device).
The offset correcting section 106 further performs for example filtering for removing cochannel interference and adjacent channel interference as required.
The OFDM time domain signal processed by the offset correcting section 106 is supplied to the symbol timing regenerating section 107 and the FFT section 108.
The symbol timing regenerating section 107 is supplied with the OFDM time domain signal from the offset correcting section 106, and is also supplied with transmission line characteristics from the transmission line characteristic estimating section 109.
The symbol timing regenerating section 107 generates a symbol synchronizing signal indicating an FFT interval as an interval of the OFDM time domain signal as an object of the FFT operation in the FFT section 108 on the basis of the OFDM time domain signal from the offset correcting section 106 and the transmission line characteristics from the transmission line characteristic estimating section 109. The symbol timing regenerating section 107 then supplies the symbol synchronizing signal to the FFT section 108.
The FFT section 108 extracts the OFDM time domain signal (sample value of the OFDM time domain signal) of the FFT interval from the OFDM time domain signal from the offset correcting section 106 according to the symbol synchronizing signal supplied from the symbol timing regenerating section 107, and performs the FFT operation. An OFDM signal representing data transmitted on a subcarrier, that is, a transmission symbol on the IQ constellation is obtained by the FFT operation on the OFDM time domain signal.
The OFDM signal obtained by the FFT operation on the OFDM time domain signal is a signal in a frequency domain, and will hereinafter be referred to also as an OFDM frequency domain signal.
The FFT section 108 supplies the OFDM frequency domain signal obtained by the FFT operation to the transmission line characteristic estimating section 109 and the transmission line distortion correcting section 110.
The transmission line characteristic estimating section 109 estimates transmission line characteristics with respect to each subcarrier of the OFDM signal using the pilot signal arranged as shown in FIG. 2 in the OFDM frequency domain signal from the FFT section 108. The transmission line characteristic estimating section 109 then supplies transmission line characteristic data (estimated value of the transmission line characteristics) indicating the transmission line characteristics to the symbol timing regenerating section 107 and the transmission line distortion correcting section 110.
The transmission line distortion correcting section 110 performs distortion correction that corrects the OFDM frequency domain signal from the FFT section 108 for amplitude and phase distortion that the subcarriers of the OFDM signal undergo in a transmission line, using the transmission line characteristic data from the transmission line characteristic estimating section 109 (for example corrects distortion of the OFDM frequency domain signal by dividing the OFDM frequency domain signal by the transmission line characteristic data). The transmission line distortion correcting section 110 supplies the OFDM frequency domain signal after the distortion correction to the error correcting section 111.
The error correcting section 111 subjects the OFDM frequency domain signal from the transmission line distortion correcting section 110 to necessary error correction processing, that is, for example deinterleaving, de-puncturing, Viterbi decoding, spread signal removal, and RS (Reed-Solomon) decoding. The error correcting section 111 outputs decoded data obtained as a result of the error correction processing.
FIG. 4 shows an example of configuration of the transmission line characteristic estimating section 109 in FIG. 3.
The transmission line characteristic estimating section 109 in FIG. 4 includes a pilot extracting section 201, a reference signal generating section 202, an arithmetic section 203, a time direction characteristic estimating section 204, and a frequency direction characteristic estimating section 205. The frequency direction characteristic estimating section 205 includes a phase adjusting section 206, a phase offset calculating section 207, an upsampling section 208, and an interpolating filter 209.
The OFDM frequency domain signal supplied from the FFT section 108 to the transmission line characteristic estimating section 109 is supplied to the pilot extracting section 201.
The pilot extracting section 201 extracts the pilot signal arranged as shown in FIG. 2, for example, from the OFDM frequency domain signal from the FFT section 108. The pilot extracting section 201 supplies the pilot signal to the arithmetic section 203.
The reference signal generating section 202 generates a pilot signal identical to that included in the OFDM signal by the transmitting device. The reference signal generating section 202 supplies the arithmetic section 203 with the pilot signal as a reference signal serving as a reference in estimating transmission line characteristics with respect to the pilot signal included in the OFDM frequency domain signal.
In the ISDB-T standard and the DVB-T standard, the pilot signal is obtained by subjecting predetermined data to BPSK (Binary Phase Shift Keying) modulation. The reference signal generating section 202 generates the signal obtained by subjecting the predetermined data to BPSK modulation, and supplies the signal as the reference signal to the arithmetic section 203.
Incidentally, the predetermined data that becomes the pilot signal is determined in advance for an OFDM symbol number and a subcarrier number at a position at which the pilot signal (transmission symbol of the pilot signal) is disposed.
The reference signal generating section 202 generates a pilot signal obtained by performing BPSK modulation of predetermined data determined in advance for an OFDM symbol number and a subcarrier number at a position of the pilot signal (transmission symbol of the pilot signal) extracted by the pilot extracting section 201. The reference signal generating section 202 supplies the pilot signal as the reference signal to the arithmetic section 203.
The arithmetic section 203 estimates transmission line characteristics with respect to the pilot signal (hereinafter referred to also as transmission line characteristics at the position of the pilot signal) by dividing the pilot signal from the pilot extracting section 201 by the reference signal from the reference signal generating section 202. The arithmetic section 203 supplies transmission line characteristic data indicating the transmission line characteristics to the time direction characteristic estimating section 204.
Distortion that the OFDM signal undergoes in a transmission line (distortion caused by a multipath and the like) is a multiplication of the OFDM signal. Thus, the component of the distortion that the OFDM signal undergoes in the transmission line, that is, the transmission line characteristics at the position of the pilot signal can be estimated by dividing the pilot signal from the pilot extracting section 201 by the reference signal.
The time direction characteristic estimating section 204 estimates transmission line characteristics in which interpolation in a time direction is performed using the transmission line characteristic data indicating the transmission line characteristics at the position of the pilot signal in a symbol number direction (time direction). The time direction characteristic estimating section 204 supplies transmission line characteristic data indicating the transmission line characteristics (which data will hereinafter be referred to also as time direction interpolation characteristic data) to the frequency direction characteristic estimating section 205.
Incidentally, the time direction interpolation characteristic data obtained in the time direction characteristic estimating section 204 is not only supplied to the frequency direction characteristic estimating section 205 but also supplied to the symbol timing regenerating section 107 as required. The symbol timing regenerating section 107 obtains the delay spread of a multipath or the like from the time direction interpolation characteristic data and generates the symbol synchronizing signal using the delay spread as required.
The frequency direction characteristic estimating section 205 estimates transmission line characteristics in which interpolation in a frequency direction is performed, that is, transmission line characteristics with respect to each of subcarriers of OFDM symbols, by filtering the time direction interpolation characteristic data from the time direction characteristic estimating section 204. The frequency direction characteristic estimating section 205 supplies transmission line characteristic data indicating the transmission line characteristics (hereinafter referred to also as frequency direction interpolation characteristic data) to the transmission line distortion correcting section 110.
Specifically, in the frequency direction characteristic estimating section 205, the time direction interpolation characteristic data from the time direction characteristic estimating section 204 is supplied to the phase adjusting section 206.
In the frequency direction characteristic estimating section 205, the phase offset calculating section 207 calculates a phase offset as a quantity for adjusting the phase of the time direction interpolation characteristic data from the time direction characteristic estimating section 204. The phase offset calculating section 207 supplies the phase offset to the phase adjusting section 206.
The phase adjusting section 206 adjusts the phase of the time direction interpolation characteristic data from the time direction characteristic estimating section 204 according to the phase offset from the phase offset calculating section 207. The phase adjusting section 206 supplies the time direction interpolation characteristic data after the phase adjustment to the upsampling section 208.
The upsampling section 208 generates time direction interpolation characteristic data whose data amount (number of sample values) is three times that of the original time direction interpolation characteristic data by interpolating for example two zeros as new sample values between sample values of the time direction interpolation characteristic data from the phase adjusting section 206. The upsampling section 208 supplies the time direction interpolation characteristic data to the interpolating filter 209.
The interpolating filter 209 is an LPF (Low Pass Filter) that performs filtering for interpolation in the frequency direction. The interpolating filter 209 filters the time direction interpolation characteristic data from the upsampling section 208.
The filtering by the interpolating filter 209 removes a repetitive component produced in the time direction interpolation characteristic data by the interpolation of zeros in the upsampling section 208, and provides frequency direction interpolation characteristic data indicating transmission line characteristics in which interpolation in the frequency direction is performed, that is, transmission line characteristics with respect to each of the subcarriers of the OFDM symbols.
The frequency direction interpolation characteristic data obtained in the interpolating filter 209 as described above is supplied to the transmission line distortion correcting section 110 as transmission line characteristic data to be used for correction of distortion of the OFDM signal.
FIG. 5 is a diagram of assistance in explaining the time direction interpolation characteristic data indicating the transmission line characteristics in which interpolation in the time direction is performed, the time direction interpolation characteristic data being obtained by the time direction characteristic estimating section 204 in FIG. 4 using the transmission line characteristic data at the position of the pilot signal in the arrangement shown in FIG. 2.
In FIG. 5, circular marks (white circular marks and hatched circular marks) represent a subcarrier (transmission symbol) of the OFDM signal.
The hatched circular marks in FIG. 5 represent a subcarrier whose transmission line characteristics are estimated (that has a sample value of the transmission line characteristic data (time direction interpolation characteristic data)) after processing in the time direction characteristic estimating section 204.
As shown in FIG. 5, the time direction characteristic estimating section 204 can obtain transmission line characteristics of every third subcarrier for each OFDM symbol.
FIG. 6 is a diagram of assistance in explaining the frequency direction interpolation characteristic data indicating the transmission line characteristics in which interpolation in the frequency direction is performed, the frequency direction interpolation characteristic data being obtained by the frequency direction characteristic estimating section 205 in FIG. 4 using the time direction interpolation characteristic data indicating the transmission line characteristics in which interpolation in the time direction is performed (transmission line characteristics of every third subcarrier) as indicated by the hatched circular marks in FIG. 5.
The frequency direction characteristic estimating section 205 obtains the transmission line characteristics of each of the subcarriers of an OFDM symbol (transmission line characteristics in which interpolation in the frequency direction is performed) which subcarriers are enclosed by a hatched rectangle in FIG. 6, using the time direction interpolation characteristic data in which transmission line characteristics are obtained for every third subcarrier in a subcarrier number direction (frequency direction).
Specifically, in the frequency direction characteristic estimating section 205, the phase adjusting section 206 adjusts the phase of the time direction interpolation characteristic data from the time direction characteristic estimating section 204 according to the phase offset supplied from the phase offset calculating section 207. The phase adjusting section 206 supplies the time direction interpolation characteristic data after the phase adjustment to the upsampling section 208.
The upsampling section 208 generates time direction interpolation characteristic data whose data amount is three times that of the original time direction interpolation characteristic data by interpolating two zeros as new sample values between sample values of the time direction interpolation characteristic data from the phase adjusting section 206. The upsampling section 208 supplies the time direction interpolation characteristic data to the interpolating filter 209.
That is, the time direction interpolation characteristic data supplied from the phase adjusting section 206 to the upsampling section 208 is a series of sample values indicating the transmission line characteristics of every third subcarrier as shown in FIG. 5, the series of sample values being obtained in the time direction characteristic estimating section 204.
Therefore, in the case of the time direction interpolation characteristic data supplied from the phase adjusting section 206 to the upsampling section 208, there are two subcarriers whose transmission line characteristics are not estimated between subcarriers whose transmission line characteristics are estimated. Thus, the upsampling section 208 interpolates two zeros as sample points of transmission line characteristics with respect to the two subcarriers whose transmission line characteristics are not estimated.
Thus, the number of zeros interpolated in the upsampling section 208 differs depending on the number of subcarriers at an interval of which transmission line characteristics are represented by the time direction interpolation characteristic data obtained in the time direction characteristic estimating section 204 as transmission line characteristic data.
When the upsampling section 208 interpolates two zeros between sample values of the time direction interpolation characteristic data from the phase adjusting section 206 as described above, the time direction interpolation characteristic data obtained as a result of the interpolation (hereinafter referred to also as zero-value interpolation characteristic data) includes a repetitive component in a time domain.
That is, the time direction interpolation characteristic data (transmission line characteristic data) is data obtained from the OFDM frequency domain signal, and is data in a frequency domain.
The time direction interpolation characteristic data and the zero-value interpolation characteristic data obtained by interpolating zeros in the time direction interpolation characteristic data are an identical signal as analog signal, and therefore the data in the time domain of the time direction interpolation characteristic data and the data in the time domain of the zero-value interpolation characteristic data are data of an identical frequency component.
In addition, the time direction interpolation characteristic data is a series of sample values indicating the transmission line characteristics of every third subcarrier. Because the interval between subcarriers is Fc=1/Tu [Hz] as described above, an interval between sample values of the time direction interpolation characteristic data as series of sample values indicating the transmission line characteristics of every third subcarrier is 3Fc=3/Tu [Hz].
Therefore, an interval between sample values of the zero-value interpolation characteristic data obtained by interpolating two zeros between sample values of the time direction interpolation characteristic data is Fc=1/Tu [Hz].
On the other hand, the time direction interpolation characteristic data whose interval between sample values is 3Fc=3/Tu [Hz] has ⅓Fc=Tu/3 [seconds] as one cycle in the time domain.
The zero-value interpolation characteristic data whose interval between sample values is Fc=1/Tu [Hz] has 1/Fc=Tu [seconds] as one cycle in the time domain, that is, has three times the cycle of the time direction interpolation characteristic data as one cycle.
The data in the time domain of the zero-value interpolation characteristic data having the same frequency component as the data in the time domain of the time direction interpolation characteristic data and having three times the cycle of the data in the time domain of the time direction interpolation characteristic data as one cycle as described above is formed by repeating the data in the time domain of the time direction interpolation characteristic data three times.
That is, FIG. 7 shows the data in the time domain of the zero-value interpolation characteristic data.
Incidentally, in order to simplify description, suppose in the following that a multipath is formed by two paths (a two-wave environment).
In FIG. 7 (as in FIG. 8, FIG. 9, and FIGS. 11 to 13 to be described later), an axis of abscissas indicates time, and an axis of ordinates indicates the power level of paths (OFDM signal).
The zero-value interpolation characteristic data (data in the time domain of the zero-value interpolation characteristic data) having a cycle of Tu [seconds] is formed by repeating a multipath corresponding to the time direction interpolation characteristic data (data in the time domain of the time direction interpolation characteristic data) having a cycle of Tu/3 [seconds] three times.
Supposing that a second (central) multipath (represented by hatching in FIG. 7) of multipaths corresponding to the time direction interpolation characteristic data repeated three times in the zero-value interpolation characteristic data is a desired multipath to be extracted as frequency direction interpolation characteristic data, obtaining the desired multipath corresponding to the frequency direction interpolation characteristic data needs removal of the other multipaths.
Accordingly, the interpolating filter 209 (FIG. 4) removes the multipaths other than the desired multipath by filtering the zero-value interpolation characteristic data. The interpolating filter 209 thereby extracts the desired multipath corresponding to the frequency direction interpolation characteristic data.
Incidentally, the zero-value interpolation characteristic data is data in the frequency domain, and the filtering of the zero-value interpolation characteristic data in the interpolating filter 209 is the convolution of the filter coefficient of the interpolating filter 209 and the zero-value interpolation characteristic data as data in the frequency domain.
The convolution in the frequency domain is a multiplication of a window function in the time domain. Therefore, the filtering of the zero-value interpolation characteristic data in the interpolating filter 209 can be expressed as a multiplication of the zero-value interpolation characteristic data (data in the time domain of the zero-value interpolation characteristic data) and a window function corresponding to the pass band of the interpolating filter 209 in the time domain.
In FIG. 7 (as in FIG. 8, FIG. 9, and FIGS. 11 to 13 to be described later), the filtering of the zero-value interpolation characteristic data is expressed as a multiplication of the zero-value interpolation characteristic data and the pass band (window function corresponding to the pass band) of the interpolating filter 209.
The cycle of the multipath repeated three times in the zero-value interpolation characteristic data is Tu/3 [seconds]. Therefore the desired multipath corresponding to the frequency direction interpolation characteristic data can be extracted by making the interpolating filter 209 for example an LPF having a band from −Tu/6 to +Tu/6 as a pass band whose bandwidth is the same as the cycle Tu/3 [seconds] of the multipath repeated three times.
Incidentally, noise included in the zero-value interpolation characteristic data (time direction interpolation characteristic data) can be reduced by adjusting the bandwidth of the pass band of the interpolating filter 209 (see Japanese Patent Laid-Open No. 2005-312027 hereinafter referred to as Patent Document 1, for example).
In addition, the phase adjusting section 206 (FIG. 4) adjusts the phase of the time direction interpolation characteristic data according to the phase offset supplied from the phase offset calculating section 207 so that the desired multipath is included within the pass band of the interpolating filter 209 as shown in FIG. 7.
Specifically, FIG. 8 shows the zero-value interpolation characteristic data obtained from the time direction interpolation characteristic data when the phase adjusting section 206 does not adjust the phase of the time direction interpolation characteristic data.
When the phase adjusting section 206 does not adjust the phase of the time direction interpolation characteristic data, a path as a part of the desired multipath (represented by hatching in FIG. 8) falls outside the pass band of the interpolating filter 209, and for example a path as a part of a multipath (on the left in FIG. 8) preceding the desired multipath is included within the pass band as pre-echo.
The phase adjusting section 206 adjusts the phase of the time direction interpolation characteristic data so that the whole of the desired multipath is included within the pass band of the interpolating filter 209.
The adjustment of the phase of the time direction interpolation characteristic data by the phase adjusting section 206 is performed by rotating a complex signal (an I-component and a Q-component) as a sample value of the time direction interpolation characteristic data according to the subcarrier number of a subcarrier corresponding to the sample value and the phase offset calculated in the phase offset calculating section 207.
Incidentally, the phase offset calculating section 207 for example obtains the delay spread of the multipath from the time direction interpolation characteristic data, and calculates the phase offset using the delay spread (see Patent Document 1, for example).